Sensing technologies, which artificially duplicate the five senses and have capabilities superior to the capabilities of organisms, will become indispensable technologies for protecting the safe, healthy, and untroubled living conditions in these diversified human societies and global environments. If an odor sensor (an artificial olfactory sensing system) that is as excellent as the odor sensor of an organism is put into practice, information that has not been used yet becomes usable, so that there is a possibility that such information is applied to robots, self-driving vehicles, medical care, and the like (refer to Nonpatent Literature 1, for example).
It is said that sensors that highly sensitively sense a few specific molecules such as an explosive-derived molecule, so-called specialist-type sensors, will be put into practice in the near future using a technology brought about by fusing biotechnologies and nano-technologies. On the other hand, as for an artificial olfaction system that sniffs out a large number of odor patterns including a mixer of plural kinds of molecules from background odors, that is, a so-called generalist-type artificial olfaction system, although the studies about the relevant biotechnology and semiconductor technology have been started, this generalist-type artificial olfaction system has not been put into practice yet.
As one of biotechnologies, a technology regarding an odor sensor that uses sensor cells each of which includes a biological olfactory receptor made artificially using biotechnology is disclosed (refer to, for example, Nonpatent Literature 2 and Patent Literature 1).
In addition, as one of semiconductor technologies, an achievement in which a response to the volatile odorous substance of the olfactory epithelium cells of a mouse is detected as an electrical signal using an ion-sensitive field-effect transistor is disclosed (refer to, for example, Nonpatent Literature 3).
Furthermore, as a technology brought about by combining a biotechnology with a semiconductor technology, a technology regarding a configuration and a method, by which an optical or an electrical response generated at the time when an olfactory receptor of an olfactory cell extracted from an organism recognizes an odor molecule is measured using a field-effect transistor, is disclosed (refer to, for example, Nonpatent Literature 2).
The technologies disclosed in Patent Literature 1 and Nonpatent Literatures 2 and 3 are olfactory sensing systems specialized for sensing specific molecules, that is, they are so-called specialist-type olfactory sensing systems. To put it concretely, only insect pheromone molecules can be detected by the technologies disclosed in Patent Literature 1 and Nonpatent Literature 2, and only VOCs (volatile organic compounds), which are biomarkers regarding cancers, can be detected by the technology disclosed in Nonpatent Literature 3.
On the other hand, a configuration for realizing a generalist-type artificial olfactory system will be explained with reference to a schematic diagram of the olfactory nerve system of a mammal typified by a human being shown in FIG. 7. When the olfactory nerve system of a mammal is roughly divided, the olfactory nerve system includes three regions, that is, (1) an olfactory epithelium 81, (2) an olfactory bulb 82, and (3) a brain 83.
There are plural kinds of olfactory cells 84 on the olfactory epithelium 81 inside a nasal cavity, and plural olfactory cells 84 of the same kind are neurologically connected to a single corresponding glomerulus 85 on an olfactory bulb 82 via axons 86. There is a loose relationship of a key and a keyhole between an odor molecule 87 and an olfactory cell 84, and signals issued by plural olfactory cells 84 which have recognized odor molecules 87 are added up at the corresponding glomerulus 85, and the firing pattern of the glomerulus 85 corresponding to the relevant odor pattern is displayed on the olfactory bulb 82.
The brain 83 identifies the relevant odor by retrieving odor memories stored in the brain itself and by comparing the retrieved memories with the firing pattern displayed on the olfactory bulb 82. Because the signals of plural olfactory cells 84 are added up at a glomerulus 85, the noises included in the signals cancel each other, so that the S/N ratios of the signals are improved.
A human being has about 400 kinds of olfactory cells 84 on the olfactory epithelium 81, in which each kind of olfactory cells issues a response signal different from response signals issued by other kinds to odor molecules 87 and the number of each kind of olfactory cells is about 1,000. Here the simplest case is considered where each olfactory cell 84 responds to a certain odor molecule 87 by issuing two different responses signals “0” and “1”. In this case, because 2400 (=2.5×10120) kinds of response signals different from each other are issued in response to the odor molecule 87, if it becomes possible to associate these odor pattern signals with odor memories, a generalist-type artificial olfactory sensing system can be realized.
In order to build such a generalist-type artificial olfactory sensing system, (1) plural kinds of olfactory cells 84 that recognizes odor molecules 87 and issues different response signals, or odor molecule sensor groups equivalent to the plural kinds of olfactory cells 84, (2) glomeruli 85 each of which adds up response signals of plural olfactory cells of the same kind, or odor signal adding-up mechanisms equivalent to the glomeruli 85, and (3) a brain 83 that compares odor patterns issued by the plural glomeruli 85 or by the plural odor signal adding-up mechanisms with odor memories, or a retrieval algorithm that carries out the function of the brain 83 on the behalf of the brain 83, and a retrieval device including the retrieval algorithm are required.
A technology regarding such a generalist-type sensor is disclosed in Patent Literature 2. In Patent Literature 2, a generalist-type olfactory sensing system is built using olfactory cells and glomeruli derived from an organism.